Update Rickshaw.Graph.js

Fixing an apparent bug in this.stackData: when we are toggling between series of different scales the data[index] access at line 149 can hit an array out-of-bounds error if there are inactive series.  Added active() to the call at line 146, to parallel the call at line 130.  This seems to fix the issue.
Merge pull request #287 from davidhc/patch-1

Update Rickshaw.Graph.js